Gremlin-1 (20.7 kDa), an antagonist of bone morphogenetic protein (BMP), is a protein consisting of 184 amino acids having a common structure with a cysteine rich region, a cysteine knot motif and members of the TGF-β superfamily. This protein is evolutionarily conserved and the human gremlin gene (GREM1) has been mapped to chromosome 15q13-q15 (Topol L Z et al., (1997) Mol. Cell Biol., 17: 4801-4810; Topol L Z et al., Cytogenet Cell Genet., 89: 79-84). Gremlin-1 is a secreted protein, and three isoforms have been reported (Topol L Z et al., J. Biol. Chem., 275: 8785-8793). Isoform 1 is the most common isoform, and isoforms 2 and 3 have deletions of amino acids 39-79 and 10-79, respectively.
Gremlin-1 forms heterodimers with BMP-2, BMP-4 and BMP-7, and thus it inhibits that bone morphogenetic protein (BMP) binds to receptors on the cell surface (Stanley E et al., (1998) Mech. Dev., 77: 173-184; Merino R et al., (1999) Development, 126: 5515-5522; Lappin D W et al., (2000) Curr. Opin. Nephrol. Hypertens., 9: 469-472). In addition, gremlin-1 plays an important role in regulating BMPs during lung, limb and kidney development as well as during neural crest cell differentiation (Lu M M et al., (2001) Dev. Dyn., 222: 667-680; Shi W et al., (2001) Am. J. Physiol. Lung Cell Mol. Physiol., 280: L1030-1039). In addition to its antagonistic effect on soluble ligands, gremlin-1 interacts intracellularly with the BMP-4 precursor protein and downregulates BMP-4-mediated signaling activity in embryonic lungs (Sun J et al., (2006) J. Biol. Chem., 281: 29349-29356). Gremlin-1 also interacts with Slit proteins, a family of secreted axonal guidance proteins, and acts as an inhibitor of monocyte chemotaxis (Chen B et al., (2004) J. Immunol., 173: 5914-5917). Recently, it was reported that gremlin-1 binds vascular endothelial growth factor receptor-2 (VEGFR2) in a BMP-independent manner and modulates angiogenesis (Mitola S et al., (2010) Blood, 116: 3677-3680). Gremlin-1 is overexpressed in various human tumors including carcinomas of the cervix, endometrium, lung, ovary, kidney, breast, colon and pancreas (Namkoong H et al., (2006) BMC Cancer, 6: 74; Sha G et al., (2009) Fertil Steril., 91: 350-358), but its role in carcinogenesis has not been studied in detail.
The present inventors have studied the tumor-related characteristics of gremlin-1, and as a result, have found that gremlin-1 interacts directly with cancer cells in a BMP- or VEGFR2-independent manner, and based on this finding, have a neutralizing antibody capable of inhibiting gremlin-1.